Why does it take so much energy to heat liquid water?
It is because of the hydrogen bonding between water molecules.
In order to cause water to increase in temperature, the energy being put into the water first needs to break hydrogen bonds between the water molecules. Hydrogen bonding in water is significant, so it takes a significant amount of energy to overcome them.
Once they are broken, further energy input will then result in increased vibration of the water molecules, and increased collisions occur between them. This results in an increased internal energy within the water, and it increases in temperature.